This invention relates to a protective coating composition. More particularly, it relates to a silicone resin coating composition which, when applied to a solid substrate, forms a protective, abrasion-resistant coating thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant, transparent plastics. The lighter weight of these plastics in comparison to glass is another advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantages of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch due to everyday contact with abrasives, such as dust, cleaning equipment and ordinary weathering. Continuous scratching and marring results in impaired visbility and poor aesthetics, and oftentimes requires replacement of the glazing or lense or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, and good dimensional stability. It is also self-extinguishing and is easily fabricated. Acrylics, such as polymethylmethacrylate, are also widely used transparent plastics for glazing.
Attempts have been made to improve the abrasion-resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225 (Misch et al.); 3,976,497 (Clark); 3,986,997 (Clark); 4,027,073 (Clark); 4,159,206 (Armbruster et al.) and 4,177,315 (Ubersax), for example, describe such compositions.
Improved such compositions are disclosed in U.S. application Ser. No. 964,910, filed Nov. 30, 1978, now abandoned and U.S. Pat. No. 4,309,319. In the former, it was discovered that, in direct contrast to the teachings of U.S. Pat. No. 3,986,997, compositions having a basic pH, i.e., 7.1-7.8, do not immediately gel but in fact provide excellent abrasion-resistant coatings on solid substrates. In the latter it was disclosed that the abrasion-resistance of silicone coating compositions is improved if a thickening agent is added to the composition.
In recent months, it has become desirable to improve the "weatherability", that is, the resistance to discoloration and breakdown due to exposure to ultraviolet (UV) radiation and moisture, offered by the coating compositions of abandoned U.S. application Ser. No. 964,910. Moreover, in view of the usefulness of these compositions in coating plastic substrates, improving the adhesion of silicone resin compositions to plastic and other materials is also desirable.
It has been discovered that such silicone coatings, especially if acidic, fail to adhere to certain plastic substrates, such as polycarbonate, and, even if prepared on the basic side of neutrality, they may adhere initially but peel after brief light aging. In U.S. Pat. No. 4,299,746, the addition of an ultraviolet light absorbing agent is suggested, but in some cases this may have a plasticizing effect, and polycarbonate seems to have a tendency to reject the coating on severe exposure. Another approach is to use a primer coat which adheres to both the silicone resin coating and the polycarbonate substrate, and which also serves as a binder for high levels of UV absorbers. The silicone resin coating is put on as a second coat over the primer coat. See, for example, Humphrey, Jr., U.S. Pat. No. 4,188,451. A further development, disclosed in commonly assigned, copending U.S. application Ser. No. 327,448, filed Dec. 4, 1981, is the employment of UV-absorbing compounds in the primer coat to give added weatherability to coated substrates.
It has now been discovered that a uniquely serviceable coating composition having a combination of properties including abrasion resistance, weatherability and compatability with conventional as well as UV-absorber-modified primer solutions is obtained by addding both a thickening agent and an ultraviolet light absorbing compound to the silicone resin compositions of abandoned U.S. application Ser. No. 964,910.